How can an investor tell if a bank is heading for danger? In the past five years, analysts have proposed all manner of financial measures. But why not analyse the words of the person running the bank? Researchers have been looking at the speech patterns of leaders such as British politicians and bank chief executives. And this has revealed a point that we instinctively know but often forget: power not only goes to the head, but also to the tongue.

More specifically, when leaders become hubristic, it generates what psychologists call “linguistic biomarkers”. Hubris has long fascinated poets, philosophers and political scientists. Four years ago David Owen, a former British foreign minister who happens also to be a psychiatrist, tried to give the idea a firmer framework by listing 14 markers of hubris. He examined dozens of British and US politicians over the past century and concluded that some leaders (such as the former British prime ministers Margaret Thatcher and Tony Blair) had succumbed to hubris in office. Others (such as John Major) had not.

Peter Garrard, a London neurology professor, has taken this analysis a step further. Dementia is known to cause changes in speech. “Meaning, emotion and attitude are communicated intentionally through language, but psychological and cognitive changes can be reflected in more subtle ways, of which a speaker remains unaware,” he says. But Dr Garrard was curious to see if power changed speech, too. So he assembled a group to analyse all the words uttered by the three British prime ministers who served from 1979 to 2007 – Thatcher, Mr Blair and Mr Major – during their questioning by British MPs in their regular parliamentary grillings.